1. Field of the Invention
The present invention relates to a focus control system used in optical recording and/or playback apparatus, and particularly to the technique of initial pull-in of a focus servo operation.
2. Description of the Prior Art
For optical recording and/or playback apparatus represented by compact disc players and video disc players, it is necessary to position the focal point (beam waist) of the reading and/or recording laser beam accurately to the recording surface of the disc, and the focus servo operation has this performance.
The focus servo operation necessitates an initial pull-in operation (will be termed simply "pull-in operation" hereinafter) at the beginning of the focusing operation. The reason for this operation is that the characteristics of focus error detection for detecting the displacement of the laser beam focus position from the recording surface have a very narrow linear range as compared with a swing motion of the disc surface and therefore the laser beam cannot be brought to the in-focus state by simply closing the servo loop (position feedback loop).
The following explains a conventional focus servo system used in an optical compact disc player based on the "three-beam, tracking generation" scheme. FIG. 1 shows the arrangement of the focus servo system used in the conventional compact disc player (CD player), in which reference numeral 1 denotes an optical recording disc, 2 is a spindle motor which rotates the disc, and 3 is an optical pickup device.
The pickup 3 is made up of a housing 31, an objective lens 32 for converging the reading laser beam to form a small light spot, a focus coil 33 which constitutes a moving member together with the objective lens 32, a suspension 34 which supports the moving member so that it is movable in the direction normal to the disc surface, and a magnetic circuit 35 including a magnet for driving the moving member in response to a current supplied to the focus coil.
The pickup 3 further includes a laser diode 36 as a light source, a diffraction grating 37 which splits the laser beam into a main beam and two subordinate beams, and an optical sensor 38 which detects the intensity of return beams from the disc 1 separately in multiple areas of its sensing surface and produces electric current outputs (the optical sensor 38 seen from above its light incident surface is shown by enlargement beneath the pickup 3). The optical sensor 38 has four light sensing areas A, B, C and D for receiving the main beam, and light sensing areas E and F for receiving the two subordinate beams separately.
Indicated by 39 is a half-mirror which deflects the laser beam from the laser diode 36 by 90.degree. toward the disc 1 and transmits the return beam from the disc 1 to the optical sensor 38. The half-mirror 39 also functions to provide astigmatism for the return beam of the main beam in proportion to the displacement of the laser beam focus position from the recording surface of the disc 1, and the state of astigmatism is detected by the divisional optical sensor 38 thereby to detect the focus error. A laser control circuit 4 which controls the optical output of the laser diode 36 is placed outside the pickup 3.
In FIG. 1, indicated by reference numerals 5 through 8 are a first through fourth current-to-voltage converters which convert the current outputs of the divisional optical sensor 38 into voltage signals. Reference numerals 9 is a first subtracter, the output of which becomes the tracking error signal. Reference numeral 10 is a first adder, the output of which is called "RF signal" and it becomes the main signal which carries reproduced information. Reference numeral 11 is a second subtracter, the output of which becomes the focus error signal. Reference numeral 12 is an in-focus state detector, reference numeral 13 is a second switch which opens or closes the servo loop. Reference numeral 14 is a loop filter which stabilizes the servo system and increases the loop gain. Reference numeral 15 is a second adder, the output of which drives the focus coil 33.
Indicated by reference numeral 16 is a sweep signal generator for producing a saw-tooth waveform, in which are included a first voltage source 161 for providing a positive voltage V.sub.1, a second voltage source 162 for providing a negative voltage -V.sub.1, a constant current source 163, a third switch 164, and a capacitor 165. Indicated by reference numeral 17 is a first switch which selects as to whether or not the output of the sweep signal generator 16 is to be fed to the second adder 15. Reference numeral 18 is a logical inverter, and reference numeral 19 is a centralized controller including a CPU, the roles of which include control of the pull-in operation.
The focus error signal produced by the second subtracter 11 has characteristics of detection as shown in FIG. 2. The focus error signifies inherently the distance between the position of focal point (focus position) of the laser beam and the recording surface of the disc, and the detected focus error is idealy related linearly with the true focus error as shown by the dashed line in FIG. 2. Actually, however, due to the conversion of the focus displacement into astigmatism and the emergence of eclipse caused by the limit in the aperture area of the objective lens 32, the sensing area of the optical sensor 38 and the width of light paths, the characteristic curve deviates from the ideal line and becomes to decline oppositely as the focus displacement grows to a certain value and eventually the detection output falls to zero, as shown by the solid line.
The S-shaped characteristic curve of FIG. 2 has in its central section a linear zone which virtually coincides with the ideal line, and this linear zone is generally defined within a width of .+-.4 to .+-.8 .mu.m. The focus servo system is designed to amplify the focus error signal and apply the signal to the focus coil 33 to move the objective lens 32 so that the focus error decreases to zero. This purpose is virtually accomplished when the servo system operates in the linear zone.
On the other hand, the compact disc 1 has a tolerant surface vibration of .+-.0.4 mm as stated by the standard of CD, which is incomparably greater than the width of linear zone. On this account, it is very difficult to bring the objective lens to the in-focus state by simply closing the focus servo loop. Specifically, when the focus servo loop is closed in a state far from the in-focus state, the focus error signal is virtually zero, i.e., the system has a zero loop gain, and the objective lens 32 is not driven to the in-focus point, or in the case of a state nearer to the in-focus state, the characteristic gradient opposite to the ideal line exerts a positive feedback on the servo system, causing the lens to be driven away from the in-focus point.
In carrying out the pull-in operation, the objective lens is moved in a wide range to find the in-focus point, with the servo loop being open, and then the loop is closed in the vicinity to the in-focus point. The principle of producing astigmatism in response to the focus displacement by the half-mirror 39 is known in the art, and the explanation thereof is omitted.
Next, the operation of the in-focus state detector 12 will be explained with reference to FIG. 3. The in-focus state detector 12 is a zero-cross comparator having hysteresis characteristics for the input focus error signal, and it produces an output signal called "FZC" (focus zero cross) signal. The waveforms of FIG. 3 are plotted along the horizontal axis which represents the distance between the focus position and the disc surface, and the input signal is derived from FIG. 2. The FZC signal goes high when the focus error signal increases across a positive threshold as the focus position approaches the disc surface, and it turns low when the focus error signal falls across the zero level at the in-focus point. Accordingly, the FZC signal indicates the in-focus state by its falling edge.
Next, the pull-in operation of the focus servo system arranged as described above will be explained with reference to FIG. 4 and FIG. 5. FIG. 4 is a flowchart showing the operation of the centralized controller 19 during the initial focus pull-in operation, and FIG. 5 is a waveform diagram showing the principal signals and switch positions during the operation.
In FIG. 4, block 100 indicates the setting of the switches before the commencement of pull-in operation, i.e., before the time point t1 in FIG. 5. At this time point, the second switch 13 is off, causing the servo loop to be open, the third switch 164 is on, causing the sweep signal generator 16 to provide the fixed output -V.sub.1, and the first switch 17 is on, causing the second adder 15 to receive the output of the sweep signal generator 16. Consequently, a negative voltage is applied to the focus coil 33, and it keeps the objective lens 32 located at the lowest end of its vertical moving range. In this system, the objective lens 32 is driven to go away from the disc surface in response to the application of a negative voltage to the focus coil 33.
When the playback start command is entered (not shown), the sequence proceeds to step 101. In step 101, a time-limit timer T.sub.1 for the pull-in operation starts time-counting. In the next step 102, the pull-in operation commences by turning off the third switch 164. This time point is t1. The constant current source 163 begins to charge the capacitor 165, and the sweep signal generator 16 has its output voltage rising linearly from -V.sub.1. At the same time, the focus coil drive voltage rises and the objective lens 32 is driven toward the disc surface (this operation is called "the sweep operation of the objective lens 32, or the focus sweep operation").
Step 103 tests whether or not the FZC signal produced by the in-focus detector 12 is high, as a procedure for detecting the in-focus state which is the falling edge of the FZC signal. In case the FZC signal stays low, it is tested in step 104 whether or not the time limit of the timer T.sub.1 has expired. If the expiration of time limit is detected, the pull-in operation terminates in failure, or otherwise the sequence returns to step 103 to repeat the detection of a high FZC signal.
When the high FZC signal is detected (at time point t2), it is tested in step 105 whether or not the FZC signal becomes low. If a low FZC signal is not yet detected, step 106 tests the expiration of the timer T.sub.1. If the expiration of time limit is detected, the pull-in operation terminates in failure, or otherwise the sequence returns to step 105 to repeat the detection of a low FZC signal. When a low FZC signal is detected in step 105 (at time point t3), step 107 turns on the second switch 13 to close the servo loop and at the same time turns off the first switch 17 to remove the output of the sweep signal generator 16 from the adder 15, and the pull-in operation completes.
At time point t3, the objective lens 32 is virtually in-focus to the disc surface. When the servo loop is closed at this time point, the objective lens 32, which may move slightly in excess (overshooting) due to the inertia, is pulled back quickly to the in-focus point by the servo action.
In FIG. 5, the lens drive voltage swings largely to the negative region immediately after the time point t3. This is a result of amplification of the focus error signal which has made an overshoot to the negative region, and the voltage produces a force for pulling back the objective lens 32 quickly. In the steady state following the time point t3, the objective lens 32 is controlled for its position so that the focus error signal becomes zero. The waveforms shown by the dashed lines after the time point t3 are the possible result of operation when the action which has been taking place up to the time point t3 continues after t3.
The foregoing pull-in operation does not always end successfully, but it can end in failure due to the contamination of the disc surface, external vibration applied to the CD player, and the like. On this account, optical recording/playback apparatus such as CD players are generally designed to repeat the pull-in operation several times so as to prevent the failure of commencement of the playback operation, although it is highly desirable to have a single successful pull-in for the quick starting of playback.
The reliability of pull-in operation, excluding such external factors as the contamination or flaws on the disc surface, is determined from the following items. 1) the width of linear zone of the focus detecting characteristics; 2) the maximum drive force (drive voltage) of the focus coil; 3) the relative velocity between the focus position (the position of objective lens) and the disc surface at the time of closure of the servo loop.
In FIG. 4 and FIG. 5, when the servo loop is closed at t3 by detecting the in-focus state accurately, the movement of the objective lens 32 beyond the in-focus point is suppressed by the servo action, as mentioned previously. Accordingly, the higher the moving speed of the objective lens 32 at t3 or the smaller the pull-in force, the farther is the turn-back position of the objective lens 32 from the in-focus point. With the turn-back position being within the linear zone of the focus error detecting characteristics, the objective lens can surely be pulled back to the in-focus point. However, in case the objective lens does not turn back from within the linear zone, the servo system does not provide an effective braking force because of a state of positive feedback due to the inverse gradient of the focus error detecting characteristics, and the probability of successful focus pull-in will fall. Accordingly, the pull-in operation is ensured by providing a large linear zone of focus detection as a braking distance.
The following describes the constraint and influence of the above-mentioned determinative factors 1)-3) of successful pull-in imposed on the system design. In regard to item 1), it is desirable to have as large linear zone of focus error detecting characteristics as possible. Although the linear zone can be increased by modifying the optical design of the pickup 3, it results in a reduced level of signals having shorter periods reproduced on the record track of the disc (the deterioration in the frequency response of signal reproduction), and therefore a drastic increase of linear zone cannot simply be done.
In regard to item 2), it is relatively easy to provide a sufficient drive force of the focus coil 33, with a possible constraint being in the case of a battery-powered CD player where the power voltage is low. In regard to item 3), it is desirable to have a sufficiently low relative velocity between the objective lens and disc surface. The determinative factors of the relative velocity in the pull-in operation will further be explained in the following.